The present invention relates to a digital communication system, and more particularly to a communication system which is suited to perform high-speed transmission above several megabits per second between an exchanger and terminals by using telephone wire as a transmission medium.
In a digital communication system, received data is acquired by discriminating a received waveform in synchronism with a reference clock pulse train. To this end, a technique for producing the reference clock pulses from the received waveform, that is, a method of extracting timing signals, becomes important. In a system for high-speed digital transmission above several megabits per second wherein telephone wire is used, signal attenuation in the telephone wire is high, and also crosstalk noise is high, so that the use of a multilevel transmission code of narrow data rate range is desirable. As to the extraction of the timings under such conditions, an LC tank circuit has been known, by way of example. In order to extract the necessary clock pulses with, for example, the partial response class-4 code, "PR4 code", the received signal is raised to the fourth power, and the biquadrate is applied to the LC tank circuit.
On the other hand, regarding digital transmission of low speed, there has been known a method wherein the received waveform is converted by an A/D converter into digital data, which are subjected to correlative operations so as to extract the timings.
A zero cross detection method has been known which is applicable to the high-speed transmission and which can be realized by comparatively simple hardware. By way of example, "IEEE NTC 1980, 65. 4" discloses a method wherein the timings of zero cross points are detected by a discriminator having a threshold level of O volt, and these signals are input to a phase locked loop "PLL", to extract the timing clock pulses. This method is extensively applied to the data transmission of 2-level codes.
Also, a method in which a PLL is used after full-wave rectification has been proposed for the purpose of applying it to the 3-level AMI code. This method is disclosed in Technical Report on Communication Systems CS81-187, the Institute of Electronics, Information and Communication Engineers in Japan.